In this situation there is no meaningful way to recognize whose Contact Details they are so, in practice you’d just have a bunch of Contact Details “floating” about your DB with no way of utilizing them. In practice I am sure there would be some way to recognize which users Contact Details they are.
Usually the issue is that somebody create Contact Details and had a data field of type text, which they use the users username. In that situation, if the app creator wants to now make a ‘live link’ by creating on the ‘user’ data type a data field of type Contact Details they can just simple make a recurring workflow that searches the Contact Details text field to correlate to the correct user and then make changes to the user by taking the found Contact Details and adding it to the ‘user’ data type.